Mh. If you look at the heroes "passive" features loosing them means loosing a lot of benefits. And I don't think they are that powerfull. You won't engage them that often in tactical battles if you compare the amount of units and the amount of planets. And at least if you want to benfit from their tactical abilities you have to leave them at your planets. So taking them into battles means, as mentioned above, loosing special benefits. I don't know if this is worth it very often. I don't think they we will see many heroes fighting except very important battles and there the enemy will have his heroes ready to engage yours. But in the end we have to wait and see.